This invention relates to adjustment of impedance values and, more particularly, to a test set for generating impedance value settings to adjust a network to yield a desired driving point impedance.
Hybrid networks are commonly used in bidirectional signal transmission systems to couple signals from a bidirectional transmission path to incoming and outgoing unidirectional transmission paths, a typical example being in coupling a 2-wire telephone bidirectional transmission cable to a 4-wire telephone transmission arrangement including two unidirectional cables. As is well known, in such arrangements it is necessary to balance the hybrid network by employing an impedance network which has a driving point impedance substantially equal to the impedance of the bidirectional transmission cable. Otherwise low transhybrid loss results which, in turn, typically results in unwanted signal reflections.
Bidirectional transmission cables of various lengths are employed in communications systems and, therefore, present a wide range of impedances. Consequently, it has been necessary to provide a precision hybrid balance network including various manual adjustments in order to generate a driving point impedance which matches the impedance of the particular bidirectional cable being connected to the hybrid network. Heretofore, such manual adjustments have relied heavily on knowledge of the characteristics of the particular bidirectional transmission cable being connected to the hybrid. Needless to say, such knowledge is not always readily available and is not always accurate.
More recently, computer programs have been developed which simulate the characteristics of the particular bidirectional cable to be connected to the hybrid and generate the desired impedance settings for the balance network. Here again, precise knowledge of the transmission cable characteristics is required to obtain optimum inpedance settings.
It has also been proposed to eliminate the need for manual adjustment of the balance networks by employing automatic balancing arrangements which continuously adjust impedance elements to effect an optimum match to the cable impedance. For the most part these continuously adjustable networks are expensive and, therefore, not practical for widespread use from an economic standpoint.